The present invention relates to organic electroluminescent devices which comprise at least one layer comprising at least one phosphorescent dopant and at least two matrix materials.
The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. No. 4,539,507, U.S. Pat. No. 5,151,629, EP 0676461 and WO 98/27136. A development in the area of organic electroluminescent devices is phosphorescent OLEDs. These have significant advantages owing to the higher achievable efficiency compared with fluorescent OLEDs.
However, there is still a need for improvement in the case of phosphorescent OLEDs. This applies, in particular, to the efficiency and the lifetime of the device.
The technical object on which the present invention is based therefore consists in the provision of a phosphorescent organic electroluminescent device which has an improved lifetime. A further object consists in the provision of a phosphorescent organic electroluminescent device which has improved efficiency.
In accordance with the prior art, electron-conducting materials, including ketones (for example in accordance with WO 2004/093207 or in accordance with the unpublished application DE 102008033943.1) or triazine derivatives (for example in accordance with the unpublished application DE 102008036982.9), are used as matrix materials for phosphorescent emitters. With ketones in particular, low operating voltages and long lifetimes are achieved, which makes this class of compound a very interesting matrix material. However, there is still a need for improvement on use of these matrix materials, as in the case of other matrix materials, in particular in respect of the efficiency and the lifetime of the device.
The prior art furthermore discloses organic electroluminescent devices which comprise a phosphorescent emitter doped into a mixture of two matrix materials.
US 2007/0252516 discloses phosphorescent organic electroluminescent devices which comprise a mixture of a hole-conducting matrix material and an electron-conducting matrix material. Improved efficiency is disclosed for these OLEDs. No effect on the lifetime is evident.
US 2007/0099026 discloses white-emitting organic electroluminescent devices, where the green- or red-emitting layer comprises a phosphorescent emitter and a mixture of a hole-conducting matrix material and an electron-conducting matrix material. Hole-conducting materials indicated are, inter alia, triarylamine and carbazole derivatives. Electron-conducting materials indicated are, inter alia, aluminium and zinc compounds, oxadiazole compounds and triazine or triazole compounds. Further improvements are also still desirable for these OLEDs.